The present invention relates to an in-line analyzer for particle size distribution in a particulate-containing flue gas or vent gas or, more particularly, to an in-line analyzer for particle size distribution in a flue gas or vent gas containing dusty and/or misty particles separately for the solid and liquid constituents.
Needless to say, any flue gases from a combustion furnace and vent gases in the gas-carrying duct from a powder-processing plant, for example, contain a considerable amount of dust, which may be soot formed by combustion of fuels or scattered powder. In addition to the dust particles, i.e. solid particles suspended in the gas, these gases sometimes contain mist which means extremely fine liquid particles or droplets suspended in the gas. Further, various chemical plants utilize a process of gas scrubbing with an aqueous alkaline solution or by spraying of brine resulting in the solid constituent such as the alkali or salt dissolved in the mist particles of the vent gas from the process. Such a dust-and/or mist-containing gas is generally called aerosol.
Various instruments working on several principles are known for the determination of the concentration and size distribution of particulates in aerosol. However, a so-called cascade impactor is the only class of such instruments by which size distribution of dust particles in a vent gas can be determined in field work as is the case in the cascade impactors developed and reported by A. A. Anderson, et al. in Journal of Bacteriology, volume 76, pages 471-484 (1958), by V. A. Marple, et al. in Journal of Aerosol Science, volume 7, pages 425-433 (1976) and by M. J. Pilat, et al. in Journal of Atmospheric Environment, volume 4, pages 671-679 (1970). The essential structure of the instrument of this type includes several particle-size classifying units in a cascade arrangement of which each unit consists of a tapered gas-ejection nozzle and an impaction disc positioned just below the gas-ejection nozzle to face the nozzle opening, the opening of a nozzle positioned higher having a larger diameter than that positioned lower. When a dust-containing gas is ejected out of the uppermost nozzle with a specified velocity at the impaction disc, the gas flow is deflected by the impaction disc while the dust particles having a diameter exceeding a certain limit or so-called cut-off size break away out of the gas flow to impinge at the impaction disc and are captured thereon due to the inertial force of the particles in proportion to the square of the particle diameter and the velocity. The gas flow freed from the dust particles of the fraction in the coarsest range of diameters is then ejected out of the second nozzle and accelerated to have a larger velocity to impinge at the second impaction disc, and so on. In this manner, each of the impaction discs captures dust particles contained in the vent gas feed, the dust particle diameters being larger on the impaction disc at a higher position than on a disc at a lower position. Therefore, the particle size distribution or the amounts of the particles in the respective fractions can be determined by demounting the impaction discs and accurately weighing each of them to determine the amount of the dust particles deposited thereon.
In an actual procedure for the measurement of dust particles according to the above described principle, an instrument containing several units each composed of a gas-ejection nozzle and an impaction disc is inserted into the flue or gas duct and the dust-containing gas is drawn therethrough at the same velocity as the flow of the flue gas by suction with a suction nozzle for a length of time and then the impaction discs having the dust particles deposited thereon and demounted from the instrument are brought to a laboratory where each of the discs is weighed to determine the amount of the respective fraction of the particles having diameters in a certain range before final calculation of the particle size distribution and concentration in the flue gas.
The cascade impactor described above is not an instrument suitable for daily or routine work in respect of the large amount of labor and time taken to obtain the final results of determination. As is understood from the principle of the method, in addition, the instrument is not suitable for the in-line measurement to speedily follow the variation of the particle size distribution and concentration of dust particles contained in a flue gas in the lapse of time. When the gas contains mist particles as in the exhaust gas at the outlet of a flue-gas desulfurization plant, moreover, the mist particles deposited on the impaction disc form a liquid layer which eventually overflows out of the disc so that no accurate determination can be performed with such a conventional cascade impactor.